Reduced skeletal loading leads to muscle atrophy and bone loss in humans and most mammals. Disuse muscle atrophy and osteoporosis represent significant clinical problem for patients during prolonged periods of immobilization and bed rest. Bears and ground squirrels are largely inactive during hibernation, but they show no loss in bone mass and less muscle atrophy than would be anticipated over such a prolonged period of physical inactivity. This suggests that hibernating mammals have unique natural adaptation to musculoskeletal disuse. Although preservation of bone mass and attenuation of muscle atrophy in hibernating bears and ground squirrels have been documented, molecular mechanisms underlying this important adaptation are not known. Our goal is to identify through transcriptional changes physiologically relevant candidate pathways and understand molecular mechanisms preventing disuse muscle atrophy and bone loss in hibernating mammals. Advantage of the next generation sequencing tools will be used to conduct large scale screens of transcriptional changes in bone and muscle comparing winter hibernating and summer active animals. Genome-wide transcriptional profiling will detect sufficiently large number of differentially expressed genes for comprehensive pathway analysis elucidating functional significance of transcriptional changes. Comparison of functional gene groups and pathways enriched by co regulated genes between two evolutionary distant species with different hibernation modes is expected to reveal general transcriptional program with untapped potential for development of improved treatment and prevention of disuse muscle atrophy and osteoporosis in non-hibernating mammals, including humans. .